A transformer coupled between an AC power system and a AC/DC or DC/AC converter is usually subject to DC bias currents which means that a certain DC magnetization in the transformer occurs. The DC bias currents may be caused by a DC component in the AC voltages on the AC side of the converter which is unavoidable due to small asymmetries in the switching control of the converter valves as well as due to small differences in the reaction, i.e. in the switching characteristics of the valves. In case of a grounded star-connected transformer, a DC bias current may also be introduced into the transformer via the transformer neutral, which occurs for example in HVDC systems (High Voltage Direct Current) with ground return. This is called external DC magnetization in the following. The DC magnetization may result in an unwanted saturation of the transformer which can create significant vibration and audible noise or even lead to overcurrents and a burn-out of the transformer.
In an article “Study on Effects of DC Current on Transformers during HVDC Systems Operated in Ground-return Mode”, Proceedings of the XIVth International Symposium on High Voltage Engineering, Bejing, China, Aug. 25-29, 2005, different methods are mentioned of how to mitigate or block a DC current flowing from an HVDC system via earth through a transformer neutral. Such methods are for example to add a resistor or a capacitor in series with the neutral or to inject a reverse compensating DC current via the neutral. In general, a DC current injected via the neutral results in a symmetrical DC magnetization in the transformer, i.e. the same magnetizing current flows in all the phases of the transformer.
In EP 0475 709 B1, an inverter control device is described which is capable of suppressing DC magnetization in a three-phase transformer in order to prevent a transformer burn-out. The transformer is connected to the AC side of an inverter and is not grounded. The inverter control device determines two correcting signals from at least two of the three phase currents on the AC side of the inverter, where the correcting signals are used to correct two reference signals which are input to a control unit to control the AC output voltages of the inverter. The inverter control device thereby regulates the imbalances or asymmetries in the three output AC voltages in such a way that the DC current component of each phase becomes zero.
In modern HVDC systems, voltage source converters are used instead of line commutated converters. In the valves of the voltage source converters, the converter valves are made up of IGBTs in anti-parallel connection with so called free-wheeling diodes while in line commutated converters thyristor valves are used. The free-wheeling diodes commutate freely and with much higher frequency than the line commutated thyristors. Since the commutation of the free-wheeling diodes is not directly controlled, it is more difficult to generate purely symmetrical output voltages with a voltage source converter than with a line commutated converter, which results in asymmetries in the AC voltages on the AC side of an AC/DC or DC/AC converter. This increases the problem with DC bias currents in the transformer connected to the converter which results in higher vibration and increased audible noise. Additionally, harmonics may occur which cause interferences in nearby telephone communication lines.